An electric tachometer is a coil of wire which passes through a uniform magnetic field and therefore produces a voltage, the voltage being an indication of the motor's speed. The coil may be attached to a moving part of the motor passing through a stationary field, or the coil may be stationary and interact with a moving magnetic field, the relative motion being measured in either case. If the motor is powered by a non-electrical source, such as steam, it is only the relative motion of the coil and the field which produces the voltage. If the motor is powered by electricity, however, changes in the current powering the motor will cause a changing magnetic field, which also will produce a voltage in the coil. This noise appears to be an indication of the speed of the motor, but is not, and must be eliminated or reduced to negligible levels.